Method for production of hollow bodies and grinding bodies so produced

ABSTRACT

A method for producing hollow grinding bodies for comminuting material to be ground uses a casting mold having a cavity and includes arranging a core in the cavity with retaining elements to produce a shell-type intermediate space. At least in the area of the intermediate space a metallic material is used as a covering for the retaining elements and fuses with the casting material. The corresponding grinding body has fused-in coverings which improve the rolling behavior of the grinding body produced as only small openings are required to remove the retaining elements. Since the fusing of the metal covering into the edge area of the openings has the effect of a composite casting, even brittle materials do not suffer from chipping.

The invention relates to a method for the production of hollow grindingbodies for the comminution of grinding stock, comprising a casting moldwith a cavity, the arrangement of a casting core in the cavity, so thata shell-like interspace for casting material is obtained, theintroduction of holding elements for the core, the application of acasing onto the holding elements and the pouring of the casting materialinto the interspace.

It is known, for the grinding of cement clinker, cement raw material,coal and similar grinding stock, to use grinding bodies which have ahollow form. The hollow form of the grinding bodies affords theadvantage of a considerable weight saving, as compared with solidgrinding bodies. One disadvantage is that, where hollow bodies areconcerned, because of the limited thickness of the shell, wear is a moreserious problem than in the case of solid grinding bodies. So thatunnecessarily high wear does not occur, the grinding bodies thereforehave to be produced in such a way that they have as low a wear aspossible. A production method, known from prior public use, for suchhollow grinding bodies is composite casting. For casting, a casting moldis used which has an inner cavity in which a casting core is arranged insuch a way that an interspace remains. Casting material is poured intothis interspace, so that a shell-like body is obtained aftersolidification. There is, then, in this case, the difficulty that thecasting core has to be fixed in the inner cavity of the casting mold.Furthermore, gases emerging from the casting core have to be discharged.It is known, for this purpose, to hold the core by means of a coresupporting structure consisting of steel tubes. The steel tubes are ofhollow form, so that gases can be discharged from the core through them.In order to prevent undesirable interactions between the core supportingstructure and the casting material, the following methods for protectingthe core supporting structure are known from prior public use:

In a first method, the core supporting structure is encased with ceramicmaterial. So that the core supporting structure can be removed from thegrinding body after the solidification of the casting material, aplurality of holes are made in the shell of the grinding body. The coresupporting structure, together with its ceramic casing, is removedthrough these. The holes which have thus occurred have a relativelylarge diameter. During grinding, the edges of these holes obstruct therolling of the grinding body. What may be referred to as edge bearingoccurs. This leads to increased loads on the grinding body, so that itis subjected to increased wear at these points. Particularly wheregrinding bodies consisting of brittle material are concerned, chippingeasily occurs in this region and ultimately may lead to a failure of thegrinding body.

In a second method, the tubes of the-core supporting structure are notencased ceramically in the region of the interspace, but, instead,remain unprotected. During the casting operation, they are exposeddirectly to the casting material. The result of this is that, during thecasting operation, there is, at least partially, a fusion of the castingmaterial with the surface of the tubes of the core supporting structure.The core supporting structure is therefore not removed after thesolidification of the casting material. It remains in the grinding body.Although edge bearing then also arises during operation, there is nolonger any chipping in this region because the core tubes are fused inthe same way as in composite casting. The disadvantage of this method isthat, on account of the fused-in core tubes the core supportingstructure can no longer be removed and is lost.

The object on which the invention is based is to provide a method of thetype initially mentioned and a grinding body produced thereby, by meansof which the abovementioned disadvantages are mitigated or avoided.

The solution according to the invention lies in the features of theindependent claims. Advantageous embodiments are the subject matter ofthe dependent claims.

According to the invention, in a method for the production of hollowgrinding bodies for the comminution of grinding stock, comprising acasting mold with a cavity, the arrangement of a casting core in thecavity, so that a shell-like interspace for casting material isobtained, the introduction of holding elements for holding the core, theapplication of a casing onto the holding elements and the pouring of thecasting material into the interspace, there is provision, at least inthe region of the interspace, for using for the casing a metallicmaterial which fuses with the casting material.

The invention is based on the notion that the holding elements for thecasting core (these are mostly tubes) are provided with a protectivecasing which consists of a metallic material, so that they can fuse withthe casting material. By contrast, the holding elements themselves canbe removed since they are protected by the protective casing againstbeing fused. Two things are achieved thereby. On the one hand, the holesoccurring due to the removal of the holding elements are relativelysmall, since only the holding elements themselves, not their casing too,need to be removed. If only because of the smaller size of the holes,the adverse effects of edge bearing are reduced. In addition, the fusionof the protective casing consisting of metallic material gives rise to agood transition in the region of the holes, so that, even whereparticularly brittle materials are concerned, chipping does not occur.This results in a particularly good load-bearing behavior of thegrinding bodies and consequently in a low wear, along with quietrunning. By metallic material being used for the casing, the advantagesof a protective casing in terms of the removability of the holdingelements can thus be combined in a surprisingly simple way with thebenefits of dispensing with the casing and the resulting fusionresembling composite casting.

It has been shown that a particularly good protective effect, on the onehand, and a particularly good fusion behavior, on the other hand, can beachieved by the thickness of the metallic casing being varied locally.An adaption to locally different parameters of the solidification andcooling process can thereby be carried out. A particularly good andlargely fault-free fusion of the casing with the casting material canthus be achieved.

It is expedient to apply the metallic casing over a length such that ithas an excess length projecting into the region of the casting coreand/or of the casting mold. Since the casing consisting of metallicmaterial is not restricted only to the region of the interspace, that isto say of the subsequent zone of fusion with the casting material, whatis achieved is that a good fusion result is attained precisely in theespecially critical surface region. The freedom from faults in thefusion increases, so that the wear behavior of the grinding body isimproved further. It has been shown to be expedient if the excess lengthamounts to between one and two thirds of the diameter of the holdingelements.

In a particularly preferred embodiment, an insulating intermediate layeris produced before the application of the metallic casing. Such anintermediate insulation achieves a better thermal separation between themetallic casing and the holding element itself. This simplifies themanagement of the method according to the invention in such a way thatthe metallic casing fuses, free of faults, with the casting material,and, on the other hand, in such a way that the holding element can beremoved as easily as possible. Furthermore, the insulating intermediatelayer has the advantage that the considerable temperature changesoccurring during the casting operation act on the holding elements toonly a minor extent, thus counteracting the risk that stresses areintroduced into the solidifying casting material due to thermallyinduced length changes of the holding elements. This improves thedimensional accuracy of the grinding body produced by means of themethod according to the invention.

The insulating intermediate layer may also be formed in that themetallic casing sits loosely on the holding elements. In this case, airacts as a particularly simple and effective insulator.

Expediently, the selected cross section both of the holding elements andof the casing is as small as possible. This allows the holes occurringafter the removal of the holding elements to be kept as small aspossible.

Preferably, in the method according to the invention, the fusion of themetallic casing with the casting material can be checked for freedomfrom faults by means of ultrasonic methods known per se. This permits aparticularly good check of the process parameters of the productionmethod. As a result, all the parameters of the production method,including the dimensions of the holding elements and of the metalliccasing, can be optimized.

The invention extends, furthermore, to a grinding body for thecomminution of grinding stock, which is in the form of a hollow body andhas in its shell orifices for holding elements for holding a castingcore, casings of the holding elements being fused in so as to border theorifices, and the fused-in casings consisting of metallic material. Onaccount of the fused-in casings consisting of metallic material, thegrinding body according to the invention has in the region of theorifices a structure resembling a composite casting. This affordsadvantageous properties in terms of edge bearing in the region of theorifices and in terms of the risk to the operating reliability of thegrinding body which results from chipping in the region of the orifices.

The invention is explained below by means of an exemplary embodiment,with reference to the accompanying drawings in which:

FIG. 1 shows a cross section through a casting mold used for the methodaccording to the invention;

FIG. 2 shows an enlarged view of a detail of the surface of a grindingbody produced according to the invention, with an orifice in the regionof a holding element, in the fused state; and

FIG. 3 a, b show views of details of the casing according to theinvention with a holding element, in the unfused state.

To explain the method according to the invention, the casting mold usedis first described. The casting mold 1 has an inner cavity 2 which is ofspherical configuration. The cavity is connected to the surroundings ofthe casting mold 1 via a plurality of passage bores 3. Furthermore, aninlet (not illustrated) for the introduction of casting material isprovided.

Within the cavity 2 is located a spherical casting core 4. The latter isarranged in the cavity 2 in such a way as to give rise, between thesurface of the casting core 4 and the surface of the spherical cavity 2,to an interspace which has the same width at every point. The interspace5 thus acquires the configuration of a spherical shell.

A core supporting structure 6 is provided in order to hold the castingcore 4 in its position within the spherical cavity 2. The coresupporting structure 6 consists of a plurality of tubes 61, 62, 63 whichare plugged into the casting core 4 and are held in the bores 3 of thecasting mold 1. The tubes 61, 62, 63 meet in the center of the sphericalcasting core 4. So that the casting core 4 can be degassed, the tubes61, 62, 63 are of hollow form. The dimensions of the tubes 61, 62, 63amount to 75 mm for the outside diameter and to 25 mm for the insidediameter.

In the region of the interspace 5, the tubes 61, 62, 63 are providedwith a casing 71, 72, 73 consisting of metallic material. The metallicmaterial may be, for example, various structural steel or boiler platequalities known per se. The encased region is in this case selected suchthat it not only runs over the region of the interspace 5, but in eachcase also extends over some distance into the casting core 4 and thecasting mold 1. In the exemplary embodiment illustrated, the length ofthis distance, also designated as an excess length, amounts to about onethird of the outside diameter of the tubes 61, 62, 63.

An enlarged illustration is given in FIG. 3 a by the example of the tube62 having the casing 72. The casing 72 surrounds the tube 62 in aring-like manner. It may sit on the tube 62 tautly or with some play. Inthe latter case, there is an air gap over a wide range of thecircumference between the outer surface of the tube 62 and the casing72. This air gap has a thermally insulating effect. What is achievedthereby is that the metallic casing 72 can fuse, free of faults, withthe casting material introduced into the interspace 5, without the riskthat the tube 62 heats up excessively and likewise also fuses. Thisensures that the tube 62 can easily be removed from the solidifiedcasting material. To achieve this effect, it is not absolutely necessaryto insulate the metallic casing 72 from the core tube 62 by means of anair gap. There may also be provision for applying an insulating materialas an intermediate layer 8 in the region between the outer surface ofthe tube 62 and the metallic casing 72′. This is illustrated in FIG. 3b. In this case, too, the advantageous thermal decoupling of themetallic casing 72′ from the core tube 62 is obtained.

To produce the grinding body, after the casting core 4 is introducedinto the casting mold 1 and positioned at the correct point by means ofthe core supporting structure 6 and the tubes 61, 62, 63 of the coresupporting structure are provided with the metallic casing 71, 72, 73according to the invention, casting material is introduced through theinlet (not illustrated) into the interspace 5 in the form of a sphericalshell, until it completely fills the latter. The casting material coolsin the casting mold 1 and solidifies. In this case, fusion occurs withthe metallic material of the casings 71, 72, 73 in such a way as to giverise to a composite zone around the core tubes 61, 62, 63. The coretubes 61, 62, 63 themselves do not also fuse. After the solidificationof the casting material, they can be removed from the grinding body thusobtained.

FIG. 2 illustrates an enlarged detail of the outer surface of thegrinding body 9 produced according to the invention. An orifice 11,through which one of the core tubes 61, 62, 63 has been removed, can beseen. Illustrated diagrammatically in the region surrounding the orifice11 is an annular zone in which the metallic casing 71, 72, 73 is fusedwith the casting material. This composite casting zone is designated bythe reference symbol 10. By virtue of the invention, the orifice 11 isvery small, and it needs to be only so large that it is sufficient forthe passage of the core tube 61, 62, 63. The casing 71, 72, 73surrounding the core tube does not need to be removed, and, according tothe invention, it is fused with the casting material. Owing to the smalldimensions of the orifice 11, edge bearing in any case occurs to only aslight extent. Moreover, because of the composite zone 10 which isobtained according to the invention, there is scarcely any chipping,even where particularly brittle materials are concerned. The grindingsphere 9 according to the invention thereby achieves outstandingoperating and wear properties.

1. A method for the production of hollow grinding bodies for thecomminution of grinding stock, comprising providing a casting mold witha cavity formed therein, arranging a casting core in the cavity toproduce a shell-like interspace adapted for receiving casting material,introducing holding elements for holding the casting core, applying acasing onto the holding elements in the region of the interspace ofmetallic material which fuses with the casting material, and pouring thecasting material into the interspace.
 2. The method as claimed in claim1, further comprising locally varying the thickness of the metalliccasing.
 3. The method as claimed in claim 1 or 2, wherein the metalliccasing is applied over a length such that an excess length of themetallic casing projects into the region of the casting core the castingmold.
 4. The method as claimed in claim 3, wherein the excess lengthamounts to between one and two thirds of the diameter of the holdingelements.
 5. The method as claimed in claim 1 or 2, further comprisingapplying an insulating intermediate layer to the metallic casing beforethe pouring of the casting material into the interspace.
 6. The methodas claimed in claim 1 or 2, wherein an air layer is produced as aninsulating intermediate layer under the metallic casing.
 7. The methodas claimed in one of the preceding claims claim 1 or 2, wherein themetallic material of the casing is structural steel or boiler plate isused as metallic material.
 8. The method as claimed in claim 1 or 2,wherein spherical grinding bodies are produced.
 9. (canceled)
 10. Thegrinding body as claimed in claim 14, the thickness of the casing varieslocally.
 11. The grinding body as claimed in claim 14 or 10, furthercomprising an intermediate layer between the casings and the holdingelements.
 12. The grinding body as claimed in claim 11, wherein themetallic material of the casings is structural steel or boiler plate.13. The grinding body as claimed in claim 10 or 14, in the form of agrinding sphere.
 14. A grinding body for the comminution of grindingstock, comprising a hollow body shell, orifices formed in the hollowbody shell receiving holding elements and a casting core held in theholding elements, the holding elements having casings of a metallicmaterial fused thereon so as to border the orifices.
 15. The grindingbody as claimed in claim 14, wherein the metallic material of the casingis structural steel or boiler plate.